The Thread Size for Screws

Unified Thread Standards

• The unified thread standard is the American standard for threaded fasteners. ANSI Standards B1.2 define the unified threads in American units. ANSI B1.3M defines the unified threads in metric units. The unified thread standards start with the letter “U.” Coarse threads are UNC or “Unified National Coarse” and extra fine threads are UNEF. ANSI B1.10 defines miniature screws, referred to as UNM or Unified National Miniature. The pitch of a threaded fastener is the angle of the thread. A screw thread micrometer or screw pitch gauge measures the pitch, but you may never need a gauge. Screw standards charts give the pitch and often the number of threads per inch.

Classes of Threads

• ANSI defines classes of threads according to a tolerance allowance, calculated by an engineering formula. Three classes of internal threads and three external thread classes identify the unified threads series. Classes 1A, 2A and 3A are external threads; 1B, 2B and 3B are internal threads. Classes 1A and 1B have the most tolerance or the loosest fit. ANSI classes 2A and 2B supply commercial products and the aerospace industry with accurate screws with predictable measurements. Classes 3A and 3B are precision screws for machines and high-quality commercial use where the screw must fit snugly.

Reading the Size

• If you have a number for a threaded fastener and don’t know what it means, you probably have the screw size. The first number is the diameter of the shaft and the second number refers to threads per inch. If you have a screw that is 3/4 - 16, it has a three-fourths-inch shaft with 16 threads to the inch. A single number preceded by the “#” symbol is a standardized screw. The American Society of Mechanical Engineers and the American Society of Testing and Materials have standardized 16 small screw sizes, starting with #0 at about three-fiftieths of an inch shaft size. A #5 screw has a one-eighth-inch shaft; #14 is one-fourth inch in diameter at the shaft, and #15 is three-eighths of an inch at the shaft. Because these screws are standard sizes, you don’t need to know the thread count per inch or the pitch.

Considerations

• Screws come in six drives -- slotted, Phillips, Robertson, star, one-way and hex. Head shapes include flat, oval, round, pan, hex and hex washer. A flat-head screw is flush with the surface or countersunk. The oval screw is partly countersunk. Other screw shapes sit on top of the material. Screws have different points as well. Type A screws have sharp points, but Type B screws have blunt points for thin metals, plywood and plastics. Teks screws are self-drilling screws with sharp points that can drill a hole in thin metals. When choosing threaded fasteners, consider the type and thickness of the material you want to fasten, the size of the screw, the composition and finish of the screw, the shape of the head and the type of drive along with the thread size.